Field of the Invention
The present invention relates generally to joining and sealing systems for plastic pipe joints and, particularly, for pipe joints formed of molecularly oriented plastic pipe which incorporate a restraint feature to assist in maintaining the integrity of the joint.
Description of the Prior Art
Pipes formed from thermoplastic materials including polyolefins such as polyethylene, polypropylene and PVC are used in a variety of industries. For example, such pipes are commonly used in municipal water and sewer applications. In forming a joint between sections of pipe, the spigot or male pipe end is inserted within the female or socket pipe end. The actual manufacture of the mating sections of plastic pipe typically involves the reforming of the end of the pipe by reheating and shaping to some desired profile to provide a means of mating with the opposing end of the next pipe. The art of forming sockets (also called bells) on plastics pipes is well established, and there are numerous processes and methods in the literature. An annular, elastomeric ring or gasket is typically seated within a groove or “raceway” formed in the socket end of the thermoplastic pipe to assist in forming a sealed pipe joint between adjoining sections of pipe. As the spigot is inserted within the socket, the gasket provides the major seal capacity for the joint.
In recent years, pipe systems employing molecularly oriented pipe and particularly the so called “PVC-O” pipe have become increasingly popular. The designation “PVC-O” stands for polyvinylchloride oriented, sometimes referred to simply as molecularly oriented pipe, or “MOP.” It is well established in the literature that molecular orientation of plastics can provide enhanced mechanical properties for plastic pipe of the type under consideration, and such materials are commonly used for plastics pipes. The molecularly oriented thermoplastic materials enhance the strength of the article in certain directions by orienting the molecules in the plastic material in such directions.
Orientation is achieved by drawing or stretching the material under appropriate conditions of temperature, such that a strain (i.e. deviation from the originally formed dimensions) is induced in the plastics material to cause alignment of the molecules, and thereafter cooling the material while drawn to lock in that strain. A number of methods have been proposed whereby this principle is applied to plastic pipes, in particular in order to enhance their strength under internal pressure by circumferential and/or axial forces, or by external forces acting on the pipeline.
For example, U.S. Pat. No. 4,428,900, shows a pipe of oriented thermoplastic polymeric material having an integral socket which is manufactured by expanding a tubular blank. The tubular blank is heated by circulation of hot water to a temperature at which deformation will induce orientation of the polymer molecules. The blank is then expanded radially outward against a mold by application of internal pressure.
U.S. Pat. No. 5,449,487, shows an apparatus and method for orienting plastic pipe. A heated pipe is oriented radially by means of a conically widening mandrel which is located downstream of the plastic extruder.
The above examples are intended merely to be illustrative of the general state of the art in the manufacture of molecularly oriented pipe.
Whether the pipeline system in question is the more ordinary PVC pipeline, or the more exotic PVC-O pipeline, it is often desirable to provide a “restrained joint” to insure that the spigot or male pipe end and the female or socket end do not separate due to internal or external forces, such as hydraulic forces that exist inside the pipeline, or external forces, such as bends in the direction of the pipeline, earthquakes or ground movement, and the like.
The problem is exacerbated in the case of PVC-O pipe joints in that the prevalent socket end is often provided with an “Anger Raceway” for receiving the sealing gasket. Since an advantage of PVC-O pipe is that it can be thinner with the same type strength as traditional PVC pipe, the Anger Raceway has a different geometry than the traditional socketed grooves provided in the more traditional “Rieber” gasket sealing systems for traditional PVC pipe. This geometry has proved to be a more difficult problem from the standpoint of providing a securely sealed joint than the traditional PVC pipe joint.
One approach to joining molecularly oriented pipe with a “restrained joint” was presented in U.S. Patent Publication No. 2011/0062700 to Corbett, Jr., assigned to the assignee of the present invention. That publication disclosed a method for joining molecularly oriented pipe in which a coupling is provided which is formed of a material other than molecularly oriented pipe, such as ordinary PVC pipe. The coupling is formed as a tubular body with a combination sealing and restraint mechanism located in each of two opposing end openings of the coupling that seal and restrain mating plain spigot ends of the molecularly oriented pipe. Because the coupling is made of a material such as ordinary PVC, the sealing and restraint mechanisms can be installed in internal grooves provided in the coupling interior during normal pipe belling operations without introducing unacceptable levels of stress or strain into the product. While providing a workable solution, that approach had the disadvantage of requiring more coupling components, adding to the cost of each joint in the pipeline system.
There are presently no other commercially available solutions to the above problem known to Applicant. The existing solutions have a limited pressure range and typically rely on cutting a notch notch or the indentation of the PVC-O pipe which is typically hard and relatively brittle. In the case case of restraint mechanisms which utilize a circumferential notch machined around the circumference of the spigot, the notch presents a weak point due to the brittle nature of the PVC-O PVC-O material.
A need continues to exist, therefore, for improved techniques for manufacturing and joining plastic pipe with restrained joints and, particularly molecularly oriented pipe such as PVC-O pipe, which techniques take into account the unique properties of these types of molecularly oriented plastic materials.